1. Field of Invention
The present invention relates to a method and apparatus for impregnating one side of a porous board, such as a gypsum board, with a precise amount of compatible phase change material. The invention also relates to a gypsum board having a single surface impregnated, substantially uniformly, with a compatible accurate amount of phase change material to increase the thermal capacity of the board.
2. Description of Prior Art
It is desirable to increase the thermal inertia of the envelope of buildings, rooms and other spaces to facilitate temperature control and to allow utilization of short duration energy sources on a longer period. For instance, during the heating season, thermal inertia stores excess solar heat reducing overheating and restores the heat at night reducing the heating demand. Therefore, an increase of the thermal inertia facilitates energy conservation. During the cooling season low cost electricity or natural cooling can be used at night to store cooling using the high thermal inertia and reducing the cooling demand of the following cooling period. Also thermal inertia reduces inside temperature variations improving comfort for the occupants.
Thermal inertia can be increased by increasing the inside mass of spaces using, for instance, heavy masonry walls. Another method is to incorporate to construction materials or components products that melt and solidify at or near the space comfort zone. Those products store a great deal of energy within a slight temperature variation during the phase change without using a large volume of these products and add only little weight to the spaces. Those products are commonly called Phase Change Materials (PCM).
Thermal inertia in spaces can be increased by incorporating a PCM to the materials used to cover the inside surface of walls and ceilings (gypsum wallboard, ceiling tiles, etc.). There are major advantages to this including a large surface area for heat transfer between the PCM and the inside air, a close contact between the storage medium and the air to cool or heat, a uniform air temperature because air is surrounded by the storage medium, storage is added without utilization of useful volume, and there is no additional cost for storage medium installation during construction. The storage medium is invisible to users and does not require any control device (passive temperature regulation).
A board, such as a gypsum wallboard, containing a PCM is described in U.S. Pat. No. 4,797,160 by Salyer. The PCM choice includes alkyl hydrocarbons having 14 or more carbon atoms and having a transition temperature between 0.degree. C. and 80.degree. C. depending on the utilization envisaged. For occupants comfort the typical transition temperature is around 18.degree. C. to 24.degree. C. There is actually no such product available commercially in part because there is no proven method to incorporate the PCM in a way to obtain an acceptable product
Five different methods are known and which have been tried to incorporate PCM into gypsum wall boards. These are:
(i) simultaneous incorporation of macro-capsules of PCM linked together and arranged in a grid and of the gypsum paste between the two finishing papers;
(ii) incorporation of macro-capsules of PCM using grooves made on the backside of dry gypsum wallboards;
(iii) incorporation of small PCM impregnated pellets during the incorporation of the gypsum paste between the finishing papers;
(iv) direct mixing of the PCM with the paste; and
(v) dipping of dry wallboards into melted PCM.
It has been observed that incorporation of capsules or pellets may reduce interval binding strength of the boards.
The PCM can be directly mixed with the gypsum paste at concentrations up to 20 percent by weight according to U.S. Pat. No. 4,797,160. A critical step in gypsum wallboard fabrication is the high drying rate of boards to force premixed starch to migrate to the gypsum-paper interface; the starch assures a good adhesion of the paper on the gypsum core. It is expected that over a certain (undetermined) amount of organic PCM in the paste, starch migration rate will be reduced and that adhesion problems will affect the quality of the product.
Finally, PCM can also be incorporated into gypsum wallboards by dipping dry boards into a melted and heated PCM for a certain amount of time. This method is simple but has some drawbacks. Impregnation rate of PCM into gypsum wallboards depends on the temperature of the board, the temperature of the PCM, the porosity of the board, the relative diffusivity of the PCM into gypsum and on the duration of the operation. In a dipping process it is difficult to control all those parameters to obtain a precise amount of PCM impregnation. Another important drawback of this method is that all the volume of the wallboard is impregnated including the face exposed of the board. This changes the appearance of the product and may case problems such as paint peeling, odors, and inflammability could occur.